Writing articles is a good way. Try Helium. It pays little, but if you manage to write a large number of articles, you'll get a good monthly income (they pay you each month for the same article).
Actually, blogging is the best way. You should give it a try.

There are plenty of online opportunities for teens out there but be careful with scam sites just read its reviews and guidelines. You are already here in mylot so use your extra time responding to any discussion because you will not just earn money here but you will also learn from the discussions. Happy mylotting! hope you can find legitimate online sites that really pay.